Melt extrusion processes and films produced thereby

ABSTRACT

The invention relates to a process for melt extruding a film layer of a thermoplastic olefin which comprises extruding the film having physical characteristics under process conditions selected by reference to the Draw ratio Dr and the Aspect ratio A such that draw resonance is avoided in which in addition the process conditions are defined by reference to the Deborah number such that the molten polymer is extruded at a Draw ratio of at least 30 in a high elasticity region at a λ c  value of characteristic time as defined herein of from 0.001 to 0.6 s, a Deborah number as defined herein in excess of 0.005 and an A value as defined herein lower than 0.8. Narrower ranges of these parameters may be defined for application of the invention to different polymer types.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of U.S. Ser. No. 08/864,420,filed May 28, 1997, which claims the benefit of U.S. ProvisionalApplication No. 60/018,520, filed May 28, 1996, the disclosures of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to improved melt extrusion processes andthe films produced thereby. The invention relates especially but notexclusively to melt extrusion processes designed to provide improvedoperating conditions and/or improved film properties.

[0004] 2. Background Information

[0005] Melt processing conditions are conventionally thought to beconstrained by phenomena such as draw resonance which leads to avariation in the draw ratio; sharkskin or surface melt fracture; orbreaking of the molten portion of the film connecting the solidifiedfilm portion and the die gap. These phenomena have generally led to alimitation of the melt processing speed, of the thickness and of thedegree of orientation which could be achieved in the resulting films.

[0006] The industry has tried to alleviate these problems in a varietyof ways including the inclusion of additives to improve surface meltfracture, the blending with a branched polymer such as LDPE and otherpolymeric or oligomeric components, as well as by modification of theextrusion apparatus using additional draw off rollers.

[0007] One route to improve draw resonance behavior is to provide thepolymer itself with more elastic behavior by branching or modificationof the polymer as described for example in Dow U.S. Pat. No. 5,272,236,U.S. Pat. No. 5,278,272, and EP-608369-A1. Low draw ratios are howeverused.

[0008] Ways of avoiding the various adverse processing phenomena aredescribed in our co-pending patent application WO93/31348.The disclosureaddresses ways of avoiding unstable draw resonance zones by appropriatecontrol of the Draw or stretch ratio, Dr, and of the Aspect ratio, A.WO93/31348 did not disclose how additional control can be exercised toplace basically non- or low elasticity molten polymer fluids intoregions where they exhibit more elastic behavior.

[0009] All the art disclosed in the above prior published applicationsand patents appears to be of lesser relevance, and is incorporatedherein by cross reference and will not be discussed in detail herein.

[0010] The invention provides means of reaching and operating in anprocess envelope where the polymer melt acts elastically, even when thepolymer is initially of a non- or low-elasticity type. The noveloperating envelope which can be reached enables polymer films to be madewhich have surprising physical properties and can be made at high linespeeds.

BRIEF SUMMARY OF THE INVENTION

[0011] According to a broad aspect of the invention there is provided aprocess for melt extruding a film layer of a thermoplastic olefin whichcomprises extruding the film having physical characteristics underprocess conditions selected by reference to the Draw ratio Dr and theAspect ratio A such that draw resonance is avoided characterized in thatin addition the process conditions are defined by reference to theDeborah number such that the molten polymer is extruded at a Draw ratioof at least 30 in a high elasticity region at a λ_(c) value ofcharacteristic time as defined herein of from 0.001 to 0.6 s, a Deborahnumber as defined herein in excess of 0.005 and an A value as definedherein lower than 0.8.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 illustrates the definition of the Draw ratio, Dr, theAspect ratio, A, and the Deborah number, De, and their constituentterms.

[0013]FIGS. 2A and 2B shows plots of the variables referred to above inthe cast extrusion of certain materials LL-3003 and ECD-350D60 asdescribed in the Examples which follow.

[0014]FIG. 3 is a 3-dimensional plot of the above variables toillustrate the extended operating envelope employed in the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0015] According to a first detailed aspect of the invention there isprovided a process for melt extruding a film layer of a polyolefincomprising at least 50 wt %, preferably at least 80 wt % of ethylenederived units which process comprises extruding the film having physicalcharacteristics under process conditions selected by reference to theDraw ratio, Dr, and the Aspect ratio, A, such that draw resonance isavoided characterized in that in addition the process conditions aredefined by reference to the Deborah number, De, such that the moltenpolymer is extruded at a Draw ratio of at least 30 in a high elasticityregion at a λ_(c) value of characteristic time as defined herein of from0.005 to 0.2 seconds at 260° C., a Deborah number as defined herein inexcess of 0.005 and an A value as defined herein lower than 0.8,preferably less than 0.2.

[0016] According to a second detailed aspect of the invention there isfurther provided a process for melt extruding a film layer of apolyolefin comprising at least 50 wt %, preferably at least 80 wt % ofpropylene derived units which process comprises extruding the filmhaving physical characteristics under process conditions selected byreference to the Draw ratio, Dr, and the Aspect ratio, A, such that drawresonance is avoided characterized in that in addition the processconditions are defined by reference to the Deborah number, De, such thatthe molten polymer is extruded at a Draw ratio of at least 30 in a highelasticity region at a λ_(c) value of characteristic time as definedherein of from 0.001 to 0.6 and especially 0.002 to 0.4 seconds at 260°C., a Deborah number as defined herein in excess of 0.005 and an A valueas defined herein lower than 0.8, preferably less than 0.2.

[0017] The parameters De, Dr and A used to define and select theextrusion conditions are defined herein in detail by reference to thedrawings, see FIG. 3. They are dimensionless. λ_(c) is expressed inseconds and changes with temperature.

[0018] The draw ratio essentially describes the intensity of theextrusion or drawing process and thereby the extent to which thecross-section of the extrudate is reduced below that of the extrusiondie orifice. The invention permits operation at very high draw ratiosand hence also at high line speeds. The invention gives a relativelythin cross-section product after extrusion is completed. As to thespatial arrangement or geometry of the extrusion machinery, the highdraw ratio at high extrusion speeds is associated with a small airgap(the distance between the extrusion orifice and the location where theextrudate is solidified, such as the cooling drum surface in the castfilm process). The airgap is represented in the defining formulae whichfollow by “X”. For a cast extrusion process the Draw ratio is defined tobe the ratio of the film velocity at the chill roll to the film velocityat the die exit, i.e. Uf/Uo. The Draw ratio approximates the ratio ofthe initial film thickness at the die orifice to the thickness of theextruded film.

[0019] The Aspect ratio represents in more defined form the geometry ofthe extrusion process. Apart from the airgap X, the Aspect ratio isinversely proportional to the width of the die represented by half thedie width “Lo” in the formulae. The Aspect ratio is the ratio of X andLo. For cast extrusion the airgap height is taken from the die exit tothe contact point between the polymer web and the chill roll. It may bemeasured with a caliper. The invention is preferably at low aspectsratios and is associated with small airgaps X. The consequence of thepractice of the invention is that an extruded film becomes considerablythinner upon extrusion by the high draw ratio but that its width is notreduced (the phenomenon referred to often as neck-in) by anywhere nearthe same amount because of the small airgap. In other words the highdraw ratio leads primarily to a desirable reduction in the thickness ofthe extruded layer.

[0020] The Deborah number De includes as a parameter λ_(c) and so isproportional with the elasticity displayed by the extruded materialunder the prevailing extrusion conditions. The Deborah number alsoincludes as a parameter the extrusion speed Uo. The λ_(c) and extrusionspeed parameters combine in the invention to give a high line speed forextrusion in a more elastic area of rheological polymer behavior. Theλ_(c) value can be determined by the methodology described in the priorpublished WO96/31348 and is then referred to as the old λ_(c) value.However, the invention can be better defined to achieve its purpose ofstable operation at high speed by defining the λ_(c) parameterdifferently; it is then referred to as the new λ_(c) value.

[0021] The new λ_(c) value is determined by fitting the curve of thedynamic viscosity, η*, vs. frequency of oscillation, ω, with a modifiedCross equation (Cross, 1966), such that:${\eta^{*}(\omega)} = \frac{\eta_{0}}{1 + {\left( {\lambda_{c}\omega} \right)m}}$

[0022] where η₀ is the zero-shear rate viscosity and m a power lawindex. The Cross equation is applied to the results of the dynamicmeasurements described in WO 96/31348, p5. The resulting characteristictime, is the reciprocal of the frequency that corresponds to thetransition from Newtonian to power-law regimes.

[0023] For the purpose of material characterization the frequency spanover which the fit is carried out must be of about the same length forthe set of polymers to be compared and is typically from 0.01 to 250rad/s.

[0024] The fitting is done using the Innovative Rheological Interfacesoftware (IRIS registered Trade mark) version 5.0 developed at theUniversity of Massachusetts Amherst, USA by Winter and co-workers. Forthe purposes of this invention, λ_(c) is a direct output of “generaldata regression module” in the IRIS software.

[0025] Preferably De is from De_(min)<De<De_(max), where De_(min) andDe_(max) are lower and upper bounds for the Deborah number. These aredefined as:${De}_{\min} = \frac{b - \sqrt{b^{2} - {4\left( {0.5 + {3 \times 10^{- 2}{Dr}} - {3.6 \times 10^{- 4}{Dr}^{2}} - A} \right)a}}}{2a}$and ${{De}_{\max} = \frac{1}{{Dr} - 1}},$

[0026] with b=31Dr−596−0.37Dr², a=5020Dr−1.1×10⁵57.5Dr² for 40≦Dr≦100and A≦1.

[0027] The modified definition of λ_(c) helps to better define thestable operating envelope for the invention. For a given Draw ratio, thestretching force increases while the Deborah number increases. Theextrusion can be that of a self supporting film or of a layer in amultilayer film, the other layers of which may or may not be extrudedunder the conditions arising out of the invention.

[0028] Suitably if the polymer comprises predominantly ethylene derivedunits, has a Melt Index, as measured by ASTM 1238, condition E, (“MI”)of 30 g/10 min or less, preferably 20 g/10 min or less, even morepreferably 8 g/10 min or less, even more preferably 4 g/10 min or less.Preferably the MI is at least 0.2, especially at least 0.5. Thecharacteristic time, λ_(c), of 0.005 seconds at 260 C., more preferably0.01 and drawing said polyolefin at a draw ratio Dr of 30 or more,preferably 35 or more and using an aspect ratio A of 0.8 or less,preferably 0.2 or less.

[0029] Preferably the polyolefin is a polyethylene copolymer having adensity of from 0.90 to 0.94, especially 0.910 to 0.935, a melt index offrom 0.2 to 20, especially from 0.5 to 15 and a molecular weightdistribution as defined herein of from 1.5 to 6, especially from 2 to 5.

[0030] Advantageously the polyethylene copolymer has an Melt Index Ratio(MIR) determined by the ratio of I₂₁, over I₂ as per ASTM test of lessthan 35, preferably less than 30 and has a low internal energy ofactivation. That implies that the polymer is one which will basically bequite low-elastic in behavior under conventional extrusion conditionsand which will lack modification, such as long chain branching, toenhance its elasticity during extrusion.

[0031] The polymer may also be a polypropylene homo or copolymer havinga melt flow rate determined by an ASTM test of from 1 to 30 especiallyfrom 4 to 20. The relaxation time or the characteristic time which issynonymous at 260° C. is from 0.001 to 0.6 seconds especially from 0.02to 0.4 seconds.

[0032] Preferred ethylene based polymers that may be processed accordingto the invention preferably have a characteristic time of 0.005 secondsat 260 C. or less. Examples include all polymers that are drawn duringformation into a product. Further examples include but are not limitedto polyolefins which are homopolymers or copolymers of C₂ to C₃₀a-olefins (for the purpose of this invention ethylene is defined to bean a-olefin). Likewise polyolefins such as linear low densitypolyethylenes, high density polyethylene, very low density polyethylene,narrow molecular weights distribution (Mw/Mn) polymers such as thoseavailable under the trade name EXACT TM from Exxon Chemical Company inHouston Tex. may be used. Additional preferred polymers that may beprocessed according to the invention include polymers of polar monomers.Preferred examples include polyesters and copolymers of ethylene orpropylene with a polar monomer. Specific preferred examples includenylon polymers, polyethylene terephtalate polymers, ethylene vinylacetate polymers, polyvinyl chloride polymers, ethylene acrylatepolymers and the like.

[0033] Particularly preferred examples include Escorene LL-3003 a linearlow density polyethylene available from Exxon Chemical Company inHouston, Tex. that has a melt index of 3.2 g/10 min and a density of0.9175 and Exceed ECD350D60 a polyethylene also available from ExxonChemical Company that has a melt index of 1.0 g/10 min and a density of0.917.

[0034] The invention is adapted for use at and is preferably used athigh draw ratios such as a draw ratio as defined herein varies from 40to 150. The process is especially useful in a cast extrusion process inwhich the airgap X as herein defined is from 1 to 20 inches, especiallyfrom 1.5 to 6 inches.

[0035] Conventionally when extrusion lines are to be operated at highline speeds, the desired geometry is established and then slowly theoutput (amount of film extruded per unit time) and line speed areincreased until the target is reached. A considerable part of theoperating envelope provided by the invention is best reached by adifferent method so as to avoid draw resonance instability or otherfatal disturbances which would prevent the high line speeds from beingreached. Thus preferably the invention also provides a process forstarting up a process for melt extruding a polyolefin into a film at ahigh draw ratio while avoiding instability which comprises the steps of:

[0036] a) commencing at a low output of less than 50% of the finaltarget output and a low line speed at less than 60% of the final targetline speed;

[0037] b) increasing the output towards the target output such that Deis increased to between 0.0015 and 0.0028 while changing the line speedby less than 5% towards the final line speed;

[0038] c) increasing the line speed without substantially reducing theoutput to obtain a draw ratio of up to 40; and

[0039] d) reducing the output to obtain a draw ratio in excess of 40while De is maintained between De min and De max as herein defined.

[0040] By operating in this way the unstable operating zone and rupturezone in FIGS. 2A and 2B as described in the Examples can be avoidedwhile high line speeds and Draw ratios can be reached.

[0041] The process of the invention also leads to a film or film layerwhich has a high degree of orientation in its machine (drawing)direction but has by comparison a low degree of stretch in thetransverse direction. This leads in turn to surprisingly high transversetensile and tear strengths which can be used to obtain a balance ofproperties not otherwise achievable. The invention thus thirdly providesa film layer prepared by a melt extrusion process which has a thicknessof from 3 to 75 microns, a molecular weight distribution of from 2 to 4,and a CDBI as defined herein of from 50 to 90%, which has a machinedirection tensile strength as defined herein of from 50 MPa to 120 MPaand a transverse direction tensile strength higher than 45 MPa.

[0042] Preferably the transverse direction Elmendorf tear strength ishigher than 500 g/mil.

EXAMPLES

[0043] The following polymers were melt extruded into film:

[0044] a) Escorene LL-3003 a polyethylene derived of ethylene and1-hexene sold by Exxon Chemical Company having an MI of 3.2 and adensity of 0.9175 The grade is made in a gas phase reactor using atitanium chloride catalyst with an aluminum alkyl activator

[0045] b) EXCEED ECD-350D60 a polyethylene grade sold by Exxon ChemicalCompany, having an MI of 1.0, a density of 0.917 and made using ametallocene catalyst with methyl alumoxane as activator in a gas phaseprocess using hexene-1 as comonomer.

[0046] The polymers were extruded on a Black Clawson cast extruder linein which the extruded film is deposited on a cooled drum under thefollowing conditions:

[0047] The die width is 107 cm and the airgap between die orifice anddrum surface is 8.1 cm. The extrusion temperature measured in the meltat the extruder exit is 440 and 470° F. for Examples A and Brespectively. The air-knife pressure was adjusted to a low 0.1 psi and0.5 psi for Examples A and B respectively. The chill roll temperaturewas set and controlled to about 81° F.

[0048] The stable region can be reached by the method best explained byreference to FIG. 2A. At (a) in the Figure extrusion is started at a lowoutput of less than 50% of the final target output, actually at 100lbs/h with the line speed being actually 460 ft/min i.e. less than 60%of the final target line speed. At (b) the output is increased towardsthe target output increasing the Deborah number to about 0.025 while theline speed is kept at an actual speed of 460 ft/min, less than a 5%change towards the final line speed. At (c) line speed is increased to938 ft/min, increasing the Draw ratio Dr in the process, withoutsubstantially reducing the output so keeping the Deborah number constantto obtain a draw ratio of up to 40. Finally at (d) the output is reducedso decreasing the Deborah number and increasing the Draw ratio to obtaina Draw ratio of 100 well in excess of 40.

[0049] Overall the result of the operation is that the polymer is takenpast the region in which draw resonance occurs and then the output speedis increased in a manner to avoid the rupture region in which themelt-strength becomes insufficient to maintain the molten portion of theextrudate in the airgap (see FIG. 2B for the rupture region which alsoexists for FIG. 2A but which is not shown).

[0050] The films prepared in the stable operating region had thefollowing characteristics: TABLE LL-3003 ECD350 ECD350 FIG. 2B Drawratio 80 46 61 Output (lbs/h) 293 408 408 Line speed ( ft/mm) 938 500700 Melt temp (° F./° C.) 440/227 470/243 470/243 Shrinkage % MD 84 7882 Shrinkage % TD −41 −27 −34 Gauge(average) 0.41 0.86 0.65 Elmendorftear MD 359 254 214 Elmendorf tear TD 737 800 1350 Oxygen vaportransmission ratio 894 650 640 Dart Impact (g/mil) 148 112 Ultimatetensile strength MD (psi) 14390 8204 Ultimate tensile strength TD (psi)7157 31300 Birefringence 16.5

[0051] The films show a stress strain behavior which indicates that theplateau region can be controlled at the higher line speeds employed asshown in the figures.

[0052] Films of the invention may be less prone to transverse tearfailure when placed under longitudinal stretch when used a stretch filmlayer. The stress strain curve for the film may show a plateau regionwhich is either lowered relative to conventionally extruded films ofsimilar materials or extended so to permit a higher level overall ofstretch. The films may also be pre-stretched to a greater extent thanhitherto with attendant benefits in pallet stretch film. Highpre-stretch levels reduce materials cost while maintaining the holdingforce for the pallet. The invention can also be used to make primaryfilm which is subsequently post-oriented below the crystalline meltingpoint. Such film would have superior tensile strength and thicknesscompared to conventionally extruded films.

We claim:
 1. A film layer prepared by a melt extrusion process, the filmlayer having a thickness of from 3 to 75 microns, a molecular weightdistribution of from 2 to 4, a machine direction tensile strength offrom 50 MPa to 120 MPa and a transverse direction tensile strengthhigher than 45 MPa.
 2. The film layer of claim 1 , wherein the filmlayer has a thickness of from 3 to 75 microns, a molecular weightdistribution of from 2 to 4, a machine direction tensile strength offrom 55 MPa to 100 MPa and a transverse direction tensile strengthhigher than 45 MPa.
 3. The film layer of claim 1 , wherein the meltextrusion process is a melt extrusion of a thermoplastic olefin havingat least 80% by weight of ethylene derived units, a melt index of from0.2 to 8, and a melt index ratio of less than
 30. 4. The film layer ofclaim 1 , wherein the film layer has a density of from 0.910 to 0.935g/cm³ and a transverse Elemendorf tear strength greater than 500 g/mil.5. A film layer prepared by a process comprising: (a) selecting athermoplastic polyolefin having a characteristic time (λ_(c)) determinedby fitting a curve of dynamic viscosity (η*) vs. frequency ofoscillation (ω) such that η*(ω)=η₀/1+(λ_(c)ω)^(m),  where η₀ is the zeroshear viscosity and m is a power law index, and said λ_(c) is a value offrom 0.001 to 0.6 s; (b) setting the aspect ratio (A) of the air gaplength (X) to one-half an extrusion die orifice width (L₀) to lower than0.8, where X is defined as the distance from said die orifice of a meltextruder to the location on a chill roll where extrudate of saidpolyolefin is solidified; (c) extruding the film through said extrusiondie orifice onto said chill roll while maintaining a ratio (Dr) of filmvelocity at said chill roll (u_(f)) to film velocity at said die orifice(u_(o)) of at least 30; and (d) maintaining the product (De) of saidλ_(c) times the ratio of said film velocity at the die orifice (u₀) tosaid air gap length (X) at a value greater than 0.005.
 6. The film layerof claim 5 , wherein the film layer has a thickness of from 3 to 75microns, a molecular weight distribution of from 2 to 4, a machinedirection tensile strength of from 50 MPa to 120 MPa and a transversedirection tensile strength higher than 45 MPa.
 7. The film layer ofclaim 5 , wherein the film layer has a thickness of from 3 to 75microns, a molecular weight distribution of from 2 to 4, a machinedirection tensile strength of from 55 MPa to 100 MPa and a transversedirection tensile strength higher than 45 MPa.